ISOTOPIC BIOGEOCHEMISTRY OF DISSOLVED ORGANIC NITROGEN - A NEW TECHNIQUE AND APPLICATION

We present a new technique for isolating and isotopically characterizi ng dissolved organic nitrogen (DON) for non-marine waters, delta(15)N values for DON from lacustrine samples and data suggesting that this t echnique will be applicable to marine samples. Our technique involves preconcentration of DON via rotary evaporation and removal of dissolve d inorganic nitrogen (DIN) via dialysis using a membrane that retains material above 100 Da. Results demonstrate quantitative removal of DIN , complete recovery of DON (95% or greater) and retention of isotopic integrity (isotope effect less than 0.4 parts per thousand) for a solu tion containing a DON standard (tripeptide) and DIN in deionized water . Reproducibility of carbon and nitrogen isotope values and elemental concentrations is demonstrated for DOM from Chefswet Basin, Lake Super ior and Grand Traverse Bay, Lake Michigan. The applicability of this t echnique to marine samples is suggested by demonstrating 99% removal o f DIN from a sample of Gulf Stream water amended with ammonium and nit rate. (C) 1997 Published by Elsevier Science Ltd. All rights reserved.